From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751675AbdLAAJW (ORCPT ); Thu, 30 Nov 2017 19:09:22 -0500 Received: from LGEAMRELO11.lge.com ([156.147.23.51]:51739 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750898AbdLAAJV (ORCPT ); Thu, 30 Nov 2017 19:09:21 -0500 X-Original-SENDERIP: 156.147.1.121 X-Original-MAILFROM: minchan@kernel.org X-Original-SENDERIP: 10.177.220.163 X-Original-MAILFROM: minchan@kernel.org Date: Fri, 1 Dec 2017 09:09:19 +0900 From: Minchan Kim To: Andrew Morton Cc: Waiman Long , Dave Chinner , Vladimir Davydov , Johannes Weiner , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock Message-ID: <20171201000919.GA4439@bbox> References: <1511965054-6328-1-git-send-email-longman@redhat.com> <20171129135319.ab078fbed566be8fc90c92ec@linux-foundation.org> <20171130004252.GR4094@dastard> <209d1aea-2951-9d4f-5638-8bc037a6676c@redhat.com> <20171130124736.e60c75d120b74314c049c02b@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171130124736.e60c75d120b74314c049c02b@linux-foundation.org> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 30, 2017 at 12:47:36PM -0800, Andrew Morton wrote: > On Thu, 30 Nov 2017 08:54:04 -0500 Waiman Long wrote: > > > > And, from that perspective, the racy shortcut in the proposed patch > > > is wrong, too. Prefetch is fine, but in general shortcutting list > > > empty checks outside the internal lock isn't. > > > > For the record, I add one more list_empty() check at the beginning of > > list_lru_del() in the patch for 2 purpose: > > 1. it allows the code to bail out early. > > 2. It make sure the cacheline of the list_head entry itself is loaded. > > > > Other than that, I only add a likely() qualifier to the existing > > list_empty() check within the lock critical region. > > But it sounds like Dave thinks that unlocked check should be removed? > > How does this adendum look? > > From: Andrew Morton > Subject: list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix > > include prefetch.h, remove unlocked list_empty() test, per Dave > > Cc: Dave Chinner > Cc: Johannes Weiner > Cc: Vladimir Davydov > Cc: Waiman Long > Signed-off-by: Andrew Morton > --- > > mm/list_lru.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff -puN mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix mm/list_lru.c > --- a/mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix > +++ a/mm/list_lru.c > @@ -8,6 +8,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -135,13 +136,11 @@ bool list_lru_del(struct list_lru *lru, > /* > * Prefetch the neighboring list entries to reduce lock hold time. > */ > - if (unlikely(list_empty(item))) > - return false; > prefetchw(item->prev); > prefetchw(item->next); > > spin_lock(&nlru->lock); > - if (likely(!list_empty(item))) { > + if (!list_empty(item)) { > l = list_lru_from_kmem(nlru, item); > list_del_init(item); > l->nr_items--; If we cannot guarantee it's likely !list_empty, prefetch with NULL pointer would be harmful by the lesson we have learned. https://lwn.net/Articles/444336/ So, with considering list_lru_del is generic library, it cannot see whether a workload makes heavy lock contentions or not. Maybe, right place for prefetching would be in caller, not in library itself.